This reason could be interesting among others: America's Real Criminal Element: Lead[1]
[1] http://www.motherjones.com/environment/2013/01/lead-crime-li...
[1] http://www.motherjones.com/environment/2013/01/lead-crime-li...
"The case against: The lead theory has the same problem as the abortion theory: in the 1990s, even people who had been exposed to lead as children started committing fewer crimes. That indicates that while lead exposure may well have been a factor, it isn't the "real answer" that it's often characterized as."
Indeed! When you click on the target, you get this URL,
http://www.vox.com/cards/crime-rate-dropcrime-rate-drop/lead...
But if you load that URL, either directly or by reloading, you get a 404.
> in the 1990s, even people who had been exposed to lead as children started committing fewer crimes
Maybe people also just commit less crime as they age?
A big point of evidence that I didn't see in the Vox or Mother Jones piece is that crime rates seemed to decline in different areas corresponding to when they stopped using leaded gasoline.
"Reyes discovered that this reduction wasn't uniform. In fact, use of leaded gasoline varied widely among states, and this gave Reyes the opening she needed. If childhood lead exposure really did produce criminal behavior in adults, you'd expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too. Conversely, in states where it declined quickly, crime would decline quickly. And that's exactly what she found.
Meanwhile, Nevin had kept busy as well, and in 2007 he published a new paper looking at crime trends around the world (PDF). This way, he could make sure the close match he'd found between the lead curve and the crime curve wasn't just a coincidence. Sure, maybe the real culprit in the United States was something else happening at the exact same time, but what are the odds of that same something happening at several different times in several different countries?
Nevin collected lead data and crime data for Australia and found a close match. Ditto for Canada. And Great Britain and Finland and France and Italy and New Zealand and West Germany. Every time, the two curves fit each other astonishingly well. When I spoke to Nevin about this, I asked him if he had ever found a country that didn't fit the theory. "No," he replied. "Not one.""
There are lots of studies on how environmental and chemicals causes these illness that criminals seems to share and they are related to undeveloped/malfunctioning control centres on the brain.
Government policies to avoiding population exposure to poison and at the same time psychiatric/neurologic advances on treatment and drugs makes a stronger case for that theory.
Update: missing 'c' in centres